1. Technical Field
The present invention relates to electronic devices, and more particularly to a thermocouple and methods of fabricating thermocouples.
2. Discussion of Related Art
Thermocouples measure temperature. Thermocouples include two dissimilar metals, joined together at one end, which produce a small unique voltage for a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.
There are various problems that reduce component survival, service lifetime and accuracy for embedded thermocouples in harsh thermal environments in the presence of oxidizing or other reactive gases. These problems include failure of the thermal barrier coating including cracking and spalling, oxidation of the thermocouple, and temperature measurement errors caused by the use of non-standard thermocouple alloy compositions.
Referring to FIG. 1, thin film thermocouples are constructed by overlapping alloy layers 101 and 102 over another to form a junction 103 disposed on a substrate 104. This can be done with a set of masks, one for each leg or by direct write processes. In either case, the junction region has twice the thickness d of the film in the individual leg conductors 101 and 102. The added thickness has several possible detrimental effects. If the thermocouple is buried under a thermal barrier coating the thicker junction region can act as a mechanical tripping point particularly under thermal cycling or other thermo-mechanical stress. Higher stresses build up in thicker coatings so the junction tends to be the point at which delamination of the junction from the substrate initiates. Furthermore, the region where the top coating passes over the edge of the bottom layer is a high stress region prone to cracking or other failure modes. In the manufacture of thermal flux sensors, several thermocouples are embedded in a thermal insulator at different depths. With the overlapping junction design the junctions must be offset from each other in the horizontal plane because the thickness build up is too great if two or more junctions are located one above the other. The heat flux can be more accurately measured with the low profile thermocouples of this invention because they can be located at the same point so they are not affected by lateral temperature gradients.
Therefore, a need exists for a system and method of fabricating thermocouples having durable construction and accurate operation.